Fix type violation in libkrad

Fix type violation in libkrad

remote.c uses casts to cover up a signature difference between
iterator() and krad_packet_iter_cb.  The difference is unimportant in
typical platform ABIs, but calling the function this way is undefined
behavior (C99 6.3.2.8).  Fix iterator() to conform to
krad_packet_iter_cb and remove the casts.